The nine staff at the firm’s Poole office matched five numbers and the bonus ball in the Lotto draw on April 7.
The syndicate include a receptionist, site staff and quantity surveyors and has been playing as a group for 14 years.
Syndicate leader Derek Middleton, 60, who is a director at Stepnell, told the Bournemouth Echo: “We play online every week and I often put on up to five weeks at a time.
“When I received a message alert to say we had won £70,191 at the weekend, I had to ask Hazel, my wife, to check it as I literally couldn’t believe my eyes.”
Middleton waited until Monday morning to tell the rest of the group. He said: “I wanted to tell everyone, personally, face to face. It was really exciting to be able to give everyone such good news.
“We work in a tough industry and to be able to tell our staff they had won the money was a great feeling.”